Steel plate for hot stamping

ABSTRACT

A steel plate for hot stamping contains, in % by mass, C: 0.25% or more and 0.4% or less, Si: 1.05% or more and 1.4% or less, Mn: 0% or more and 1.4% or less, Cr: 0.6% or more and 3.0% or less, P: 0% or more and 0.03% or less, S: 0% or more and 0.02% or less, Al: 0.01% or more and 1% or less, N: 0% or more and 0.01% or less, B: 0.0005% or more and 0.005% or less, Ti: 0.005% or more and 0.1% or less, and iron and inevitable impurities as remainder. This steel plate for hot stamping exhibits excellent hardness stability in addition to a balance between strength and toughness as a relational expression of [C]2/9[Si]+7/9[Mn]+8/9[Cr]−7/4&gt;0 is satisfied.

TECHNICAL FIELD

The present invention relates to a steel plate for hot stamping.

BACKGROUND ART

In recent years, there has been a demand for improvement in collisionsafety of motor vehicles, and in association with this, there has been ademand for a further increase in strength of steel plates for hotstamping used in parts required to exhibit rigidity of motor vehicles.However, when the strength of steel plate is improved, the lowtemperature toughness deteriorates and the balance between strength andtoughness is thus lost. In order to cope with this problem, Non PatentLiterature 1 proposes that the balance between strength and toughness ofa steel plate is improved by refining the former austenite grains afterhot stamping.

In hot stamping, the cooling velocity inside the steel plate maydecrease by an increase in the die temperature and the clearance betweenthe die and the steel plate. When the cooling velocity of the steelplate is equal to or lower than the critical cooling velocity, softphases such as ferrite and bainite precipitate and the hardness of thesteel plate thus decreases. In particular, as the cooling velocity at atemperature equal to or lower than the Ms point decreases, autotempering is promoted and this causes a decrease in hardness of thesteel plate.

In Non Patent Literature 2, a change in cooling velocity when changingthe clearance between the die and the steel plate is examined and it hasbeen indicated that the cooling velocity decreases to about 15° C./swhen this clearance is 0.4 mm.

As described in Non Patent Literature 1, there is a method in which thecrystal grains of steel are refined as a general structure designtechnology of steel plates for hot stamping and this method makes itpossible to obtain a steel plate having an excellent balance betweenstrength and toughness. As a method for refining the crystal grains,there is a method in which elements such as Nb, Ni, and Ti are added,but the economical efficiency of the steel plate becomes poor in thiscase. A steel plate having refined crystal grains exhibits poorhardenability and thus lacks hardness stability.

In order to solve this problem, it is also considered to improve processproblems that cause a decrease in hardness, such as an increase in dietemperature and clearance between the die and the steel plate. However,in that case, it is required to repeatedly modify the die and prepare aspecial die, and this requires a great deal of labor and cost. Hence, inthe conventional steel plates for hot stamping, there is a problem thatit is difficult to obtain a member (molded product) having an excellentbalance between strength and toughness and excellent hardness stabilitywithout increasing labor and cost.

CITATION LIST Non Patent Literature

-   Non Patent Literature 1: Kazuo Hikida et al., “Development of TS    1800 MPa Grade Hot Stamping Steel Sheet” Materia Vol. 52, No. 2,    2013, pp. 68-70-   Non Patent Literature 2: Katsuji Nakashima, “Hardening Technology of    Steel by Die Quenching and Application to Body Parts” CAMP-ISIJ Vol.    17 2004, pp. 980-983

SUMMARY OF INVENTION

An object of the present invention is to provide a steel plate for hotstamping which can provide a molded product which exhibits excellenthardness stability in addition to the balance between strength andtoughness while suppressing increases in labor and cost in the hotstamping process.

A steel plate for hot stamping according to an aspect of the presentinvention contains,

in % by mass,

C: 0.25% or more and 0.4% or less,

Si: 1.05% or more and 1.4% or less,

Mn: 0% or more and 1.4% or less,

Cr: 0.6% or more and 3.0% or less,

P: 0% or more and 0.03% or less,

S: 0% or more and 0.02% or less,

Al: 0.01% or more and 1% or less,

N: 0% or more and 0.01% or less,

B: 0.0005% or more and 0.005% or less,

Ti: 0.005% or more and 0.1% or less, and

iron and inevitable impurities as remainder. This steel plate for hotstamping exhibits excellent hardness stability in addition to a balancebetween strength and toughness as a following relational expression (1)is satisfied, where [C] denotes a C content, [Si] denotes a Si content,[Mn] denotes a Mn content, and [Cr] denotes a Cr content.

[Math.  1] $\begin{matrix}{{\lbrack C\rbrack + {\frac{2}{9}\lbrack{Si}\rbrack} + {\frac{7}{9}\lbrack{Mn}\rbrack} + {\frac{8}{9}\lbrack{Cr}\rbrack} - \frac{7}{4}} > 0} & (1)\end{matrix}$

According to the present invention, it is possible to provide a steelplate for hot stamping which can provide a molded product which exhibitsexcellent hardness stability in addition to the balance between strengthand toughness while suppressing increases in labor and cost in the hotstamping process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating the relation between absorbed energy in aCharpy impact test when a flat plate is hardened using a die andhardness when hardening is performed at a cooling velocity of 10° C./s.

FIG. 2 is a diagram schematically illustrating a hot stamping process.

FIG. 3 is a schematic diagram illustrating the respective dimensions ofa test piece used in a Charpy pendulum impact test.

FIG. 4 is a schematic diagram illustrating the respective dimensions ofa test piece used in a hardness test.

DESCRIPTION OF EMBODIMENT

Hereinafter, a steel plate for hot stamping according to an embodimentof the present invention will be described in detail.

(Steel Plate for Hot Stamping)

The steel plate for hot stamping according to the present embodimentcontains,

in % by mass,

C: 0.25% or more and 0.4% or less,

Si: 1.05% or more and 1.4% or less,

Mn: 0% or more and 1.4% or less,

Cr: 0.6% or more and 3.0% or less,

P: 0% or more and 0.03% or less,

S: 0% or more and 0.02% or less,

Al: 0.01% or more and 1% or less,

N: 0% or more and 0.01% or less,

B: 0.0005% or more and 0.005% or less,

Ti: 0.005% or more and 0.1% or less, and

iron and inevitable impurities as remainder.

This steel plate for hot stamping exhibits excellent hardness stabilityin addition to a balance between strength and toughness as a followingrelational expression (1) is satisfied, where [C] denotes a C content,[Si] denotes a Si content, [Mn] denotes a Mn content, and [Cr] denotes aCr content.

[Math.  1] $\begin{matrix}{{\lbrack C\rbrack + {\frac{2}{9}\lbrack{Si}\rbrack} + {\frac{7}{9}\lbrack{Mn}\rbrack} + {\frac{8}{9}\lbrack{Cr}\rbrack} - \frac{7}{4}} > 0} & (1)\end{matrix}$

In order to obtain a steel plate for hot stamping which is excellent inboth the balance between strength and toughness and the hardnessstability, the present inventors have conducted extensive studies on thecomponent composition of steel plate. From the description in Non PatentLiterature 2, it has been expected that the cooling velocity of a normalmember fluctuates in a range of 30° C./s to 10° C./s in the hot stampingprocess due to the clearance between the die and the steel plate and anincrease in the die temperature. For this reason, the present inventorshave focused on suppression of the variation in hardness even when thecooling velocity fluctuates in addition to the balance between strengthand toughness, and conducted detailed investigations on the componentsystem of steel plate for achieving this. As a result, the presentinventors have newly found out that the balance between strength andtoughness and hardness stability can be both achieved by adjusting thebalance among the contents of C, Si, Mn, and Cr so that the relationalexpression (1) is satisfied as well as each component composition in asteel plate satisfies the above range, and thus conceived the presentinvention.

First, each component composition in the steel plate for hot stampingaccording to the present embodiment will be described in detail.

[C (Carbon): 0.25% by Mass or More and 0.4% by Mass or Less]

The C content determines the strength of the steel plate after diecooling. In order to obtain sufficient strength of the steel plate, theC content is 0.25% by mass or more, preferably 0.255% by mass or more,more preferably 0.260% by mass or more.

However, when the C content is excessive, the strength of the steelplate after hot rolling may increase and this may lead to crackingduring cold rolling and deterioration in weldability. Hence, the Ccontent is 0.4% by mass or less, preferably 0.38% by mass or less, morepreferably 0.36% by mass or less.

[Si (Silicon): 1.05% by Mass or More and 1.4% by Mass or Less]

Si contributes to the hardness stability of the steel plate byincreasing the temper softening resistance. Si also has an effect ofpreventing scale peeling off after die cooling when the surface of thesteel plate is not plated. In order to exert these effects, the Sicontent is 1.05% by mass or more.

On the other hand, Si facilitates the generation of retained austenite(y) and promotes a decrease in yield strength (YS) and segregation ofMn. Hence, the Si content is 1.4% by mass or less, preferably 1.35% bymass or less.

[Mn (Manganese): 0% by Mass or More and 1.4% by Mass or Less]

Mn is one of the important elements contained in the steel plate for hotstamping according to the present embodiment and contributes to anincrease in the strength of the steel plate after die cooling byenhancing the hardenability of the steel plate. In order to exert thiseffect, the Mn content is preferably 0.5% by mass or more, morepreferably 0.6% by mass or more, still more preferably 0.8% by mass ormore.

On the other hand, in the investigations to achieve both the strengthand toughness of the steel plate after die cooling, it has beenconfirmed that when Mn is excessive, coarse carbides precipitate duringdie cooling and brittle fracture is caused when shocking stress isapplied to the steel plate in a low temperature environment. Hence, theMn content is 1.4% by mass or less, preferably 1.35% by mass or less,more preferably 1.30% by mass or less.

Mn is an element that is inevitably mixed into the steel plate and it isthus difficult to set the Mn content to 0% by mass.

[Cr (Chromium): 0.6% by Mass or More and 3.0% by Mass or Less]

Cr is one of the important elements in the steel plate for hot stampingaccording to the present embodiment. In the investigations to achieveboth the strength and toughness of the steel plate after die cooling, ithas been confirmed that Cr contributes to securing of the hardness at alow cooling velocity (for example, 10° C./s) as well as suppression ofcoarse carbide precipitation during die cooling and thus suppressesbrittle fracture when shocking stress is applied to the steel plate in alow temperature environment. In order to exert these effects, the Crcontent is 0.6% by mass or more, preferably 0.8% by mass or more, morepreferably 1.05% by mass or more.

On the other hand, when Cr is excessively contained in the steel plate,the strength of the steel plate after hot rolling increases and thisleads to cracking of the steel plate during cold rolling anddeterioration in pickling property after hot rolling. Hence, the Crcontent is 3.0% by mass or less, preferably 2.5% by mass or less.

[P (Phosphorus): 0% by Mass or More and 0.03% by Mass or Less]

From the viewpoint of weldability of the member, toughness, andprevention of surface flaws, it is required to regulate the upper limitof P content. Hence, the P content is 0.03% by mass or less, preferably0.025% by mass or less, more preferably 0.02% by mass or less.

P is an element that is inevitably mixed into the steel plate and it isthus difficult to set the P content to 0% by mass.

[S (Sulfur): 0% by Mass or More and 0.02% by Mass or Less]

S forms MnS to decrease the uniformity of the Mn concentrationdistribution and also deteriorate the weldability of the steel plate.Hence, the S content is 0.02% by mass or less, preferably 0.018% by massor less, more preferably 0.015% by mass or less.

S is an element that is inevitably mixed into the steel plate as P andit is thus difficult to set the S content to 0% by mass.

[Al (Aluminum): 0.01% by Mass or More and 1% by Mass or Less]

Al is an element that acts as a deoxidizer. In order to exert thiseffect, the Al content is 0.01% by mass or more, preferably 0.015% bymass or more.

However, when Al is excessively contained in the steel plate, thehardness after die cooling decreases and excessive generation of Al₂O₃deteriorates the low temperature toughness. Hence, the Al content is 1%by mass or less, preferably 0.8% by mass or less, more preferably 0.1%by mass or less. The Al content here means the content of Al(sol.Al) ina solid solution state.

[N (Nitrogen): 0% by Mass or More and 0.01% by Mass or Less]

N is an element that is inevitably mixed into the steel plate. When N isexcessively contained in the steel plate, the amount of solid solution Bin the steel plate decreases as N forms a boride and this leads todeterioration in hardenability. Hence, the N content is 0.01% by mass orless, preferably 0.008% by mass or less, more preferably 0.005% by massor less.

[B (Boron): 0.0005% by Mass or More and 0.005% by Mass or Less]

B is an important element for improving the hardenability of the steelplate. By adding an appropriate amount of B to the steel plate, thehardenability is enhanced and this makes it possible to stably increasethe strength of the steel plate after die cooling. In order to exertthis effect, the B content is 0.0005% by mass or more, preferably0.0010% by mass or more, more preferably 0.0015% by mass or more.

On the other hand, when B is excessively contained in the steel plate, acoarse iron-boron compound precipitates and this leads to deteriorationin toughness. Hence, the B content is 0.0050% by mass or less,preferably 0.0045% by mass or less, more preferably 0.0030% by mass orless.

[Ti (Titanium): 0.005% by Mass or More and 0.1% by Mass or Less]

Ti decreases the amount of BN generated in the steel plate by formingTiN. This increases the amount of solid solution B in the steel plateand makes it possible to enhance the hardenability improving effect byB. In order to exert this effect, the Ti content is 0.0050% by mass ormore, preferably 0.010% by mass or more, more preferably 0.015% by massor more.

On the other hand, when Ti is excessively contained in the steel plate,a carbide precipitates at the crystal grain boundaries and thehardenability of the steel plate deteriorates. Hence, the Ti content is0.1% by mass or less, preferably 0.08% by mass or less, more preferably0.06% by mass or less.

The steel plate for hot stamping according to the present embodiment mayfurther contain one or more selected from the group consisting of Mo,Nb, and V or one or more selected from the group consisting of Cu and Niin addition to the above component composition. The ranges of componentcompositions of these will be described below. These elements are notessential elements in the steel plate for hot stamping of the presentinvention and may not be added.

[Mo (Molybdenum): 0% by Mass or More and 1.0% by Mass or Less]

Mo is an element that contributes to the improvement in hardenability ofthe steel plate. In order to exert this effect, the Mo content ispreferably 0.01% by mass or more. However, when Mo is excessivelycontained in the steel plate, the strength of the steel plate before hotmolding is increased. In order to prevent this, the Mo content ispreferably 1.0% by mass or less.

[Nb (Niobium) and V (Vanadium): 0% by Mass or More and 0.1% by Mass orLess]

Nb and V form fine carbides and have the effect of refining thestructure of steel by the pinning effect. V also has a secondaryhardening action by being precipitated during tempering. In order toexert these effects, the Nb and V contents are both preferably 0.0008%by mass or more.

However, when Nb and V are excessively contained in the steel plate,coarse carbides are formed and this becomes the starting point offracture to lead to deterioration in toughness. Hence, the Nb and Vcontents are both preferably 0.1% by mass or less, more preferably 0.08%by mass or less, still more preferably 0.07% by mass or less.

[Cu (Copper) and Ni (Nickel): 0% by Mass or More and 0.5% by Mass orLess]

Cu and Ni are preferably added when it is required to improve thedelayed fracture properties of the member. However, when Cu and Ni areexcessively contained in the steel plate, flaws may be generated on thesurface of the steel plate and finally on the surface of the member.Hence, it is preferable that the Cu and Ni contents are each 0.5% bymass or less and it is more preferable that the sum of the Cu and Nicontents is 0.5% by mass or less.

The steel plate for hot stamping according to the present embodimentexhibits excellent hardness stability in addition to the balance betweenstrength and toughness as the following relational expression (1) issatisfied by adjustment of the balance among the contents of C, Si. Mn,and Cr. In this relational expression (1), [C] denotes the C content (%by mass) in the steel plate for hot stamping. [Si] denotes the Sicontent (% by mass) in the steel plate for hot stamping. [Mn] denotesthe Mn content (% by mass) in the steel plate for hot stamping. [Cr]denotes the Cr content (% by mass) in the steel plate for hot stamping.

[Math.  1] $\begin{matrix}{{\lbrack C\rbrack + {\frac{2}{9}\lbrack{Si}\rbrack} + {\frac{7}{9}\lbrack{Mn}\rbrack} + {\frac{8}{9}\lbrack{Cr}\rbrack} - \frac{7}{4}} > 0} & (1)\end{matrix}$

As the relational expression (1) is satisfied as well as the respectivecomponent compositions satisfy the component ranges in the claims, thesteel plate for hot stamping according to the present embodimentexhibits excellent hardness stability as well as is a steel plate havingan excellent balance between the strength after hardening by die coolingand the low temperature toughness. Specifically, the followingrelational expressions (2), (3), and (4) are all satisfied where, A(J/cm²) denotes the absorbed energy in a Charpy impact test at −40° C.when a flat plate is hardened using a die, B(Hv) denotes the hardnesswhen the steel plate for hot stamping is heated to the austenite region,then cooled to room temperature at a cooling velocity of 10° C./s, andhardened, and C(Hv) denotes the hardness when the steel plate for hotstamping is heated to the austenite range, then cooled to roomtemperature at a cooling velocity of 30° C./s, and hardened.

[Math. 2]

B>−4.0A+627  (2)

[Math. 3]

B≥516  (3)

[Math. 4]

|C−B|≤  (4)

The relational expression (2) is an index of the balance between thestrength and toughness of the steel plate newly devised by the presentinventors and is an important concept when considering the balancebetween the strength and toughness of the steel plate for hot stamping.In the course of investigations on the balance between strength andtoughness, the present inventors have focused on the hardness when thecooling velocity is 10° C./s and the toughness after die cooling of aflat plate. In the die cooling of a flat plate, ideal cooling conditionsin which a clearance is not generated between the die and the steelplate in the hot stamping process are taken into consideration. By usingthe relational expression (2), it is possible to more faithfullyevaluate the balance between strength and toughness when the steel platefor hot stamping is processed into a member (molded product).

The graph of FIG. 1 illustrates the relation between the absorbed energyA (horizontal axis) in a Charpy impact test at 10° C. when a flat plateis hardened using a die and the hardness B (vertical axis) of the steelplate when being hardened at a cooling velocity of 10° C./s. Thestraight line (1) in this graph corresponds to the relational expression(2). The straight line (2) in this graph corresponds to an equation ofB=516.

The horizontal axis (A) of the graph of FIG. 1 assumes the toughness atthe most brittle portion of the member after die cooling. In otherwords, when a flat plate is subjected to die cooling, the die and thesteel plate are in contact with each other in an ideal state and thecooling velocity is thus high. For this reason, the strength aftercooling is high but, on the other hand, the flat plate is extremelybrittle. In other words, this horizontal axis has a meaning as toughnessat the most brittle portion when the steel plate for hot stamping ismolded into a member (molded product).

On the other hand, the vertical axis (B) of the graph of FIG. 1 assumesthe hardness of the most softened portion of the member after diecooling. As described above, in the hot stamping process, a clearancemay be generated between the die and the steel plate and the dietemperature may rise. For this reason, the member after die cooling hasa portion that is cooled at a low cooling velocity and has a lowhardness (strength). From the description in Non Patent Literature 2, itis assumed that the minimum cooling velocity during die cooling is about10° C./s. Hence, this vertical axis has a meaning as hardness (strength)at the most softened portion of the member (molded product) after diecooling. Consequently, by using these two axes, the toughness of theweakest portion when shocking stress is applied to the member afterbeing molded and the strength of the weakest portion when static stressis applied to this member can be evaluated.

Usually, in the hardness region in which B is 516 Hv or more, thestrength and toughness of a steel plate are in a trade-off relation andthus the toughness tends to deteriorate when the strength of the steelplate is improved. In other words, it is difficult to improve both thestrength and toughness of a steel plate and it is normal that thedistribution of A and B exists in the region below the straight line (1)in the graph of FIG. 1.

The straight line (2) is one index that indicates the hardnessstability. During continuous operation of the die in the hot stampingprocess, the temperature of the die may rise and a clearance may begenerated between the die and the steel plate. Due to these factors, thecooling velocity of the steel plate during hardening decreases and thehardness of the steel plate after being hardened decreases as thecooling velocity decreases. Even in the case of a steel plate in whichthe balance between strength and toughness is improved by the refinementof crystal grains, it is usually difficult for the hardness when thesteel plate is hardened in a low cooling velocity region (10° C./s) tosatisfy a range of 516 Hv or more. Hence, even in the case of a steelplate in which the balance between strength and toughness is improved bythe refinement of crystal grains, it is normal that the distribution ofA and B exists in the region below the straight line (2) in FIG. 1.

In contrast, as a result of extensive studies conducted by the presentinventors, it has been revealed that the distribution of A and B arelocated in the region above the straight lines (1) and (2) in FIG. 1 inthe steel plate for hot stamping which satisfies the relationalexpression (1). Hence, the steel plate for hot stamping according to thepresent embodiment exhibits excellent hardness stability in addition tothe balance between strength and toughness. In other words, this steelplate for hot stamping has an excellent balance between strength andtoughness that satisfies the relational expression (2) and can realize ahardness at a certain degree or more even when being cooled at theminimum cooling velocity of 10° C./s.

The relational expression (4) is another index of the hardness stabilityof steel plate. When the die temperature rises or a clearance isgenerated between the die and the steel plate during hot stamping, thecooling velocity of the steel plate may decrease and the hardness of thesteel plate after being hardened may become unstable. As describedabove, it is usually difficult to satisfy the relational expression (4)since the hardness stability decreases when the crystal grains arerefined.

In contrast, as a result of extensive studies conducted by the presentinventors, it has been revealed that a hardness exceeding 516 Hv isattained after the steel plate is hardened even in a low coolingvelocity region of 10° C./s as well as the difference in hardnessbetween a case having a cooling velocity of 30° C./s and a case having acooling velocity of 10° C./s is suppressed to 35 Hv or less in the steelplate for hot stamping in which the relational expression (1) issatisfied as well as the respective components satisfy the componentranges in the claims. 30° C./s is an ideal cooling velocity during diecooling, which has been confirmed by an experiment and the like while10° C./s is the minimum cooling velocity expected as described above. Inother words, the relational expression (4) is an index indicating thatthe difference (variation) in hardness after hardening is small betweenthe upper and lower limits of the cooling velocity assumed in hotstamping. According to the steel plate for hot stamping of the presentembodiment, it is possible to stabilize the hardness of the steel plateafter being hardened to the extent to which the relational expression(4) is satisfied regardless of the temperature rise of the die and thegeneration of clearance between the die and the steel plate.

The steel plate for hot stamping of the present invention may be a basesteel plate having a surface not subjected to a plating treatment or aplated steel plate having a surface subjected to a plating treatment.

(Method for Manufacturing Steel Plate for Hot Stamping)

Next, a method for manufacturing the steel plate for hot stampingaccording to the present embodiment will be described.

First, a slab manufacturing process is performed. In this process, aslab is obtained by melting steel according to a conventional method,pouring the molten steel into a mold, and performing continuous casting.In this process, the component composition of the steel is adjustedduring melting so that the compositions of the respective componentscontained in the slab satisfy the above ranges and the contents of C,Si, Mn, and Cr satisfy the relational expression (1).

Next, a hot rolling process is performed. In this process, the slabobtained in the above process is first disposed in a heating furnace,heated to a predetermined temperature (for example, 1200° C.), and heldat the heating temperature for a predetermined time (for example, 30minutes).

Next, the heated slab is placed upstream of the hot rolling line.Thereafter, the slab is rolled into a steel plate having a predeterminedthickness by allowing the slab to sequentially pass through between therolls of the rolling stands of the rough rolling mill and the finishingrolling mill and allowing the slab to flow downstream. Thereafter, thesteel plate after being hot-rolled is cooled to a predeterminedtemperature in a cooling apparatus and then wound by a coiler.

Next, a cold rolling process is performed. In this process, the scale(oxides of iron) generated on the surface of the steel plate in the hotrolling step is first washed off with an acid (pickling) and then thehot-rolled steel plate is further rolled so that the thickness furtherdecreases. Specifically, the hot-rolled steel plate after beingsubjected to pickling is allowed to pass through between the rolls ofthe rolling stands so that the hot-rolled steel plate is furtherthinned. The cold-rolled steel plate obtained by the above processes isthe steel plate for hot stamping according to the present embodiment.

(Hot Stamping)

Next, hot stamping performed using the steel plate manufactured by theabove processes will be described with reference to FIG. 2. First, asteel plate for hot stamping 1 manufactured by the above processes isheated in a predetermined heating furnace 2 to a temperature equal to orhigher than the austenite transformation temperature. Thereafter, thesteel plate for hot stamping 1 after being heated is disposed betweendies 3 and 4 and press-molded into a desired shape by the dies 3 and 4.At this time, the steel plate for hot stamping 1 is cooled by cominginto contact with the dies 3 and 4, and hardening is performed at thesame time as molding. Thereafter, the steel plate after being hardenedis taken out from the dies 3 and 4 as a molded product 5 (moldedmember).

The molded product 5 has the same component composition as that of thesteel plate for hot stamping 1 according to the present embodimentdescribed above and is one in which the balance among the contents of C,Si, Mn, and Cr is adjusted so that the relational expression (1) issatisfied. Hence, the molded product 5 exhibits excellent hardnessstability in addition to the balance between strength and toughness andcan be utilized in various applications including members for motorvehicles.

The outline of the above-described embodiment is as follows.

A steel plate for hot stamping according to the present embodimentcontains,

in % by mass,

C: 0.25% or more and 0.4% or less,

Si: 0.05% or more and 1.4% or less,

Mn: 0% or more and 1.4% or less,

Cr: 0.6% or more and 3.0% or less,

P: 0% or more and 0.03% or less,

S: 0% or more and 0.02% or less,

Al: 0.01% or more and 1% or less,

N: 0% or more and 0.01% or less,

B: 0.0005% or more and 0.005% or less,

Ti: 0.005% or more and 0.1% or less, and

iron and inevitable impurities as remainder. This steel plate for hotstamping exhibits

excellent hardness stability in addition to a balance between strengthand toughness as a following relational expression (1) is satisfied,where [C] denotes a C content, [Si] denotes a Si content, [Mn] denotes aMn content, and [Cr] denotes a Cr content.

[Math.  1] $\begin{matrix}{{\lbrack C\rbrack + {\frac{2}{9}\lbrack{Si}\rbrack} + {\frac{7}{9}\lbrack{Mn}\rbrack} + {\frac{8}{9}\lbrack{Cr}\rbrack} - \frac{7}{4}} > 0} & (1)\end{matrix}$

The steel plate for hot stamping may contain, in % by mass, one or moreselected from the group consisting of

Mo: 0% or more and 1.0% or less,

Nb: 0% or more and 0.1% or less, and

V: 0% or more and 0.1% or less.

The steel plate for hot stamping may contain, in % by mass, one or moreselected from the group consisting of

Cu: 0% or more and 0.5% or less, and

Ni: 0% or more and 0.5% or less.

Examples

Hereinafter, the present invention will be described in more detailbased on examples. However, the present invention is not limited to thefollowing examples, it is also possible to carry out the presentinvention by adding changes within a range that is compatible with theabove-mentioned and below-mentioned gist, and all of them are includedin the technical scope of the present invention.

<Manufacture of Steel Plate for Hot Stamping>

First, a slab was manufactured by melting steel (the remainder beingiron and inevitable impurities) having the component composition shownin Nos. 1 to 17 in the following Table 1. This molten slab was heated to1200° C., held for 30 minutes, and then hot-rolled. The finishingtemperature was set to 900±20° C., and the finishing plate thickness wasset to 2.8 mm. Thereafter, the hot-rolled steel plate was cooled to awinding temperature (CT temperature) at a cooling velocity of 20° C./sto 30° C./s, held at 650° C. for 30 minutes, and then cooled in thefurnace. Thereafter, the hot-rolled steel plate was subjected topickling and cold-rolled so that the steel plate had a thickness of 1.4mm.

<Charpy Impact Test>

First, the cold-rolled steel plate fabricated according to the aboveprocedure was cut and hardened. Hardening was performed under thefollowing conditions by a die quench method using a flat platesimulating a die (testing machine: JIS Charpy impact tester (300J)).

[Hardening Conditions]

Steel plate dimensions before hardening: 1.4 mm×70 mm×150 mm

Steel plate temperature: 900° C.

Steel plate temperature holding time after steel plate reaches 900° C.:100 seconds

Cooling time: about 15 seconds

Die quench start temperature: 700° C.

Die quench load: 2000 kgf

Bottom dead center holding time: 30 seconds

Next, a Charpy pendulum impact test was performed using the cold-rolledsteel plate after being subjected to the hardening. This test wasperformed in conformity with JIS 2242 “Charpy impact test method formetal materials” except the dimensions of the test piece. The dimensionsof the test piece used in the present test are as follows. The referencenumerals indicating the respective dimensions correspond to thereference numerals illustrated in FIG. 3.

[Test Piece Dimensions]

Test piece height h1: 10 mm±0.05 mm

Test piece length L: 55 mm±0.6 mm

Test piece width b: 1.4 mm±0.05 mm

Notch shape: V notch

Notch angle: 45°±2°

Notch bottom radius: 0.25 mm±0.025 mm

Height under notch h2: 8 mm±0.05 mm

Angle between test piece longitudinal direction and notch symmetryplane: 90°±2°

Angle between adjacent surfaces eliminating fracture surface: 90°±2°

A test piece having the above dimensions was disposed in liquid nitrogenadjusted to have a temperature of −40° C.±1° C. and held for at least 10minutes. Thereafter, the test piece was taken out from the liquidnitrogen and placed on a support, and an impact was made on the testpiece. At this time, the time until the impact was made after the testpiece was placed on a support was set to 5 seconds or less.

A JIS Charpy impact tester (300J) was used as a tester, and an impactblade having a radius of 2 mm was used. The number of test pieces wastwo, and the average value of two measured values was used forevaluation.

<Evaluation on Scale Adhesive Property>

Hardening was performed by the die quench method under the sameconditions as those in the Charpy impact test described above and thenthe degree of scale peeling off on the surface of the steel plate wasvisually confirmed to evaluate the adhesive property of scale. It wasevaluated as “◯” when the area ratio of the surface of the steel platein which scale peeling off occurred was 14% or less, and it wasevaluated as “x” when the area ratio exceeded 14%.

<Hardness Test>

First, the cold-rolled steel plate fabricated according to the aboveprocedure was processed into a test piece having a shape illustrated inFIG. 4. In FIG. 4, L1 is 10 mm, L2 is 2 mm, L3 is 1.4 mm, L4 is 0.7 mm,L5 is 3 mm, and L6 is 1 mm. Hardening was performed using this testpiece under the following conditions.

[Hardening Conditions]

Rate of temperature rise when converting to austenite: 10° C./s

High temperature holding: held at 900° C. for 100 seconds

Cooling velocity: constant cooling from 900° C. to room temperature at10° C./s or 30° C./s

A hardness test was performed using the test piece after being subjectedto the hardening in conformity with the “Vickers hardness test method”prescribed in JIS Z 2244. In this test, five points were measured at thepositions to be the ¼ plate thickness from the surface of the test pieceat a test load of 9.8 N, and evaluation was performed using the averagevalue of these.

The following Tables 1 and 2 present each of the component composition(% by mass), absorbed energy A (J/cm²) in a Charpy impact test at −40°C., Vickers hardness B (Hv) when the cooling velocity is 10° C./s,Vickers hardness C (Hv) when the cooling velocity is 30° C./s,difference in hardness (Hv) between a case having a cooling velocity of30° C./s and a case having a cooling velocity of 10° C./s, value on theleft side of the relational expression (1), value when the right side ofthe relational expression (2) is subtracted from the left side, andevaluation results on scale adhesive property for each of Nos. 1 to 17steel plates.

In the graph of FIG. 1, the respective data for Nos. 1 to 17 steelplates are plotted. The data for Nos. 1 to 9 and 14 to 17 are markedwith black circles, and the data for Nos. 10 to 13 are marked with whitecircles.

TABLE 1 Component composition (% by mass) C Si Mn Cr P S Al Cu Ni Mo NbTi N V B O No. 1 0.345 1.24 1.23 0.82 0.0100 0.0019 0.041 — — — — 0.0220.0041 — 0.0021 0.0005 No. 2 0.351 1.22 1.22 1.23 0.0090 0.0011 0.040 —— — — 0.022 0.0044 — 0.0020 0.0006 No. 3 0.337 1.22 0.81 0.82 0.00700.0018 0.041 — — — — 0.021 0.0038 — 0.0018 0.0006 No. 4 0.336 1.20 0.811.21 0.0090 0.0018 0.040 — — — — 0.021 0.0034 — 0.0019 0.0009 No. 50.342 1.22 0.22 1.21 0.0080 0.0009 0.040 — — — — 0.022 0.0044 — 0.00160.0007 No. 6 0.326 1.21 0.21 1.52 0.0090 0.0014 0.040 — — — — 0.0220.0040 — 0.0020 0.0008 No. 7 0.332 1.22 0.21 2.04 0.0100 0.0016 0.039 —— — — 0.022 0.0040 — 0.0020 0.0007 No. 8 0.294 1.19 0.85 0.83 0.01000.0010 0.038 — — — — 0.022 0.0047 0.001 0.0014 — No. 9 0.290 1.19 1.230.65 0.0100 0.0020 0.038 — — — — 0.022 0.0039 — 0.0014 — No. 10 0.2890.01 0.23 1.24 0.0040 0.0009 0.040 — — — 0.004 0.020 0.0009 0.001 0.00110.0009 No. 11 0.315 1.24 1.20 0.01 0.0040 0.0010 0.041 — — — — 0.0210.0041 — 0.0015 0.0006 No. 12 0.326 1.21 1.21 0.01 0.0040 0.0010 0.041 —— — — 0.020 0.0045 — 0.0017 0.0009 No, 13 0.266 1.19 1.21 0.01 0.00400.0012 0.041 — — — 0.004 0.020 0.0005 0.001 0.0018 0.0005 No. 14 0.2711.23 1.19 0.60 0.0040 0.0020 0.038 0.10 — — — 0.022 0.0043 — 0.00170.0007 No. 15 0.294 1.26 1.18 0.61 0.0040 0.0010 0.038 — 0.10 — — 0.0210.0047 — 0.0016 0.0007 No. 16 0.281 1.24 1.18 0.60 0.0040 0.0020 0.0380.10 0.10 — — 0.021 0.0046 — 0.0018 0.0006 No. 17 0.335 1.21 0.82 1.190.0040 0.0013 0.036 — — 0.19 — 0.021 0.0023 — 0.0019 0.0007

TABLE 2 [C] + Charpy impact test Hardness measuring test 2/9[Si] + ScaleAbsorbed energy Hardness B at Hardness C at Hardness 7/9[Mn] + B +adhesive A(J/cm²) 10° C./s(Hv) 30° C./s(Hv) difference(Hv) 8/9[Cr] − 7/44A − 627 property No. 1 24.1 594 596 2 0.56 63 ∘ No. 2 30.4 599 621 220.91 93 ∘ No. 3 30.4 562 575 13 0.22 56 ∘ No. 4 33.4 578 595 17 0.56 85∘ No. 5 35.7 548 574 26 0.11 64 ∘ No. 6 39.3 558 566 8 0.36 88 ∘ No. 731.3 570 582 12 0.83 68 ∘ No. 8 46.4 518 547 28 0.21 77 ∘ No. 9 37.5 529542 13 0.34 52 ∘ No. 10 60.7 325 498 173 −0.18 −59 x No. 11 42.0 426 564138 −0.22 −33 ∘ No. 12 39.3 458 577 119 −0.21 −12 ∘ No. 13 55.4 379 516137 −0.27 −27 ∘ No. 14 51.8 519 519 −1 0.25 100 ∘ No. 15 43.8 543 533 90.28 91 ∘ No. 16 50.9 526 531 4 0.26 103 ∘ No. 17 32.1 586 603 17 0.5588 ∘

DISCUSSION

The following can be discussed based on Tables 1 and 2.

In Nos. 1 to 9 and 14 to 17, the contents of C, Si, Mn, and Cr satisfythe relational expression (1) as well as the contents of C, Si, Mn, Cr,P, S, Al, N, B, and Ti in the steel plate each satisfy the ranges in thepresent invention. In this case, the value of “B+4A−627” is a positivevalue, the relational expression (2) is satisfied, and thus the steelplate has an excellent balance between strength and toughness. Moreover,in Nos. 1 to 9 and 14 to 17, it is “B≥516” and “C−B≤35”, the relationalexpressions (3) and (4) are also satisfied, and thus the steel platealso exhibits excellent hardness stability. This is clear from the factthat the data (black circles) for Nos. 1 to 9 and 14 to 17 exist in theregions above the straight lines (1) and (2) in the graph of FIG. 1. Theevaluation results on the scale adhesive property are all “◯”.

In contrast, in Nos. 10 to 13 that do not satisfy the requirementsregulated in the present invention, a steel plate excellent in both thebalance between strength and toughness and the hardness stability is notobtained as to be described below. As illustrated in the graph of FIG.1, the data (white circles) for Nos. 10 to 13 all exist in the regionsbelow the straight lines (1) and (2).

In No. 10, the Si content is less than 1.05% by mass and the value of“[C]+2/9[Si]+7/9[Mn]+8/9[Cr]−7/4” is a negative value, thus the value of“B+4A−627” is a negative value and the balance between strength andtoughness is poor. The hardness B when the cooling velocity is 10° C./sis less than 516 Hv, the difference in hardness between a case having acooling velocity of 30° C./s and a case having a cooling velocity of 10°C./s also exceeds 35 Hv, and the hardness stability is also poor. Theevaluation results on the scale adhesive property are also “x”.

In Nos. 11 to 13, the Cr content is less than 0.6% by mass and the valueof “[C]+2/9[Si]+7/9[Mn]+8/9[Cr]−7/4” is a negative value, thus the valueof “B+4A−627” is a negative value and the balance between strength andtoughness is poor. The hardness B when the cooling velocity is 10° C./sis less than 516 Hv, the difference in hardness between a case having acooling velocity of 30° C./s and a case having a cooling velocity of 10°C./s also exceeds 35 Hv, and the hardness stability is also poor.

It should be understood that the embodiments and examples disclosedherein are illustrative in all points and not restrictive. The scope ofthe present invention is shown not by the above description but by theclaims, and is intended to include meanings equivalent to the claims andall modifications within the scope.

1. A steel plate, comprising: in % by mass, C: 0.25% or more and 0.4% orless, Si: 1.05% or more and 1.4% or less, Mn: 0% or more and 1.4% orless, Cr: 0.6% or more and 3.0% or less, P: 0% or more and 0.03% orless, S: 0% or more and 0.02% or less, Al: 0.01% or more and 1% or less,N: 0% or more and 0.01% or less, B: 0.0005% or more and 0.005% or less,and Ti: 0.005% or more and 0.1% or less, wherein the followingrelational expression (1) is satisfied $\begin{matrix}{{\lbrack C\rbrack + {\frac{2}{9}\lbrack{Si}\rbrack} + {\frac{7}{9}\lbrack{Mn}\rbrack} + {\frac{8}{9}\lbrack{Cr}\rbrack} - \frac{7}{4}} > 0} & (1)\end{matrix}$ where [C] denotes the C content, [Si] denotes the Sicontent, [Mn] denotes the Mn content, and [Cr] denotes the Cr content.2. The steel plate of claim 1, comprising, in % by mass, one or moreselected from the group consisting of Mo: 0% or more and 1.0% or less,Nb: 0% or more and 0.1% or less, and V: 0% or more and 0.1% or less. 3.The steel plate of claim 1, comprising, in % by mass, one or moreselected from the group consisting of Cu: 0% or more and 0.5% or less,and Ni: 0% or more and 0.5% or less.
 4. The steel plate of claim 2,comprising, in % by mass, one or more selected from the group consistingof Cu: 0% or more and 0.5% or less, and Ni: 0% or more and 0.5% or less.